A conventional plastic sorting apparatus comprises two electrode plates arranged parallel with each other in a vertical direction so as to form a space of a predetermined size therebetween, the electrode plates having a high-voltage power supply connected thereto to form a separating electrostatic field therebetween, wherein plastic pieces are loaded into the separating electrostatic field from above, and the electrostatic force of the separating electrostatic field is used to vary the falling trajectories of the plastic pieces correspondingly to the charges of the plastic pieces, thereby separating the plastic pieces from one another according to their type.
However, to improve the separation precision of the conventional apparatuses, it is necessary to use long electrode plates to increase the time required for the plastic pieces to pass through the separating electrostatic field, which disadvantageously requires the size of the apparatuses to be increased. Further, the plastic pieces may adhere to the electrodes, resulting in degraded separation performance, to hinder continuous processing.
Thus, the inventors have proposed the plastic sorting apparatus shown in FIG. 9 as a technique for sorting out crushed plastic pieces. This plastic sorting apparatus is composed of a hopper 100 into which plastic pieces a and b including two or more types mixed together are loaded, a tribo-electrifying device 101 arranged below the hopper 100 to stir the plastic pieces a and b of different types to contact them with each other, thereby electrifying the plastic pieces a and b to the respective polarities and electrification amounts according to their type, and an electrostatic separating device 105 that separates the plastic pieces a and b from each other by applying a high voltage from a high-voltage power supply 104 to a drum electrode 102 and an opposing electrode 103 both arranged below the tribo-electrifying device 101, to form a sorting electrostatic field Ea.
In this plastic sorting apparatus, the tribo-electrifying device 101 frictionally contacts the plastic pieces a and b of different types with each other to tribo-electrify them to the respective polarities and electrification amounts corresponding to an electrification array. Then, when the plastic pieces a and b are fed-onto the drum electrode 102, image force is applied to the plastic pieces a and b, which are then attracted to a surface of the drum electrode 102 and carried to the sorting electrostatic field Ea. Then, in the sorting electrostatic field Ea, electrostatic force and centrifugal force are applied to the plastic pieces a and b, so that the forces acting on the plastic pieces b having the same polarity (−) as that of the drum electrode 102 are such that image force<(electrostatic force+centrifugal force). As a result, the plastic pieces b fall so as to follow falling trajectories extending from a surface of the metal drum electrode 102 to the opposing electrode 103 and are thus collected in a separation container 106. On the contrary, the forces acting on the plastic pieces a having a polarity opposite to that (−) of the drum electrode 102 are such that (image force+electrostatic force)>centrifugal force, so that the plastic pieces a remain attracted to the drum electrode 102 or fall so as to follow falling trajectories approaching the drum electrode 102 and are then collected in a separation container 107.
Methacryl resin [acrylic resin] (hereinafter referred to as “PMMA”), polyethylene resin (hereinafter referred to as “PE”), polypropylene resin (hereinafter referred to as “PP”), and vinyl chloride resin (hereinafter referred to as “PVC”) occupy about 80% of all the plastics consumed as material for plastic products. Further, polyethylene terephthalate resin (hereinafter referred to as “PET”) used for PET bottles is separately sorted out and recovered. To use these resins as recycle material for material recycling, they must be precisely (99% or more) sorted out according to their type.
Further, it is contemplated that in order to achieve a high purity and a high recovery rate with the electrostatic separating device 105 of the above described plastic sorting apparatus, a separator 108 arranged at the boundary between the separation containers 106 and 107 may pivot in directions A and B to adjust a separation position so as to improve the purity.
FIG. 10 shows the relationship between the recovery rate and purity obtained by the inventors through sorting experiments in which the angle of the separator 108 was adjusted. In this figure, the purity and the recovery rate are determined using the following equation:
Recovery rate (%)=weight (g) of plastic pieces of a target type collected in a predetermined separation container/total weight of plastic pieces of the target type loaded into an electrostatic separating device.
Purity (%)=weight (g) of plastic pieces of a predetermined type collected in a predetermined separation container/weight of all plastic pieces collected in the separation container.
FIG. 10 indicates that it is likely that an increase in purity reduces the recovery rate, whereas an increase in recovery rate reduces the purity. Thus, if plastic pieces are to be recovered according to their material, the rate at which plastic pieces are recovered becomes insufficient to make the separating operation less efficient even if the plastic pieces have an attainable purity of 99% or higher.
Further, with a lower voltage applied to the drum electrode 102 and opposing electrode 103, the field intensity of the sorting-electrostatic field Ea decreases to reduce changes of the falling trajectories of plastic pieces a and b, thereby reducing the precision (purity and recovery rate) with which the plastic pieces a and b are separated from each other. Thus, to increase the separation precision for the plastic pieces a and b, a high-voltage electrode 6 with a much higher voltage must be used to form a sorting electrostatic field Ea of an increased field intensity. In this case, however, a very high voltage must be applied to the metal drum electrode 102 and opposing electrode 103, and all members of the electrostatic separating device 105 must thus have high-voltage-resistant and highly-insulated structures, thereby substantially increasing facility costs. Consequently, it is difficult to apply a voltage exceeding a predetermined value.
Furthermore, when plastic pieces PMMA, PE, PP, and PVC are trobo-electrified using the tribo-electrifying device 101, they are electrified to their respective polarities and electrification amounts in accordance with the electrification array of (+ side) PMMA-PE-PP-PVC (− side). For example, if PE and PP, which are closely ordered in the electrification array, are contacted with each other for tribo-electrification, the PE is electrified to (+), while the PP is electrified to (−), with a small difference in electrification amount therebetween. Accordingly, if different types of plastic pieces are mixed together which are closely ordered in the electrification array and have only a small difference in electrification amount therebetween, then a problem with the above described conventional plastic sorting apparatus is that the plastic pieces cannot be sorted out with a high purity of 99% or higher and a high recovery rate.
It is an object of the present invention to provide a plastic sorting apparatus that can sort out plastic pieces according to their type with a high purity and a high recovery rate.